The development of various modern electronic devices such as calculators, digital watches, microprocessors, etc., has increased the importance of reliable, low cost display devices. Many of these modern electronic devices store information or compute information which must be displayed for rapid and frequent reading. A display is made up of a number of display elements which can be individually controlled. Characters in a display are made up of clusters of display elements. Characters are formed (for example, individual integers or letters) by turning certain of the display elements "on" and others in the "off" position. A popular and well-known example is the seven segment numeric display shown in FIG. 1. Leaving certain of these display elements "on" and others in the "off" position results in the display of a particular digit.
It is desirable to reduce cross effects between display elements (here referred to as cross-talk effects) so as to maximize optical contrast and sharpness of the displayed characters. Where individual leads and electrodes are used for both the positive and negative electrodes of each element in the display, cross-talk effects and minimized but the number of leads in the display (two per element) is very large. For example, with a seven digit display and a seven segment numeric for each digit the number of individual display elements is 49. For individual positive and negative electrodes for each display element, this relatively simple display would require 96 individual leads. It is highly desirable to reduce the number of individual leads so as to simplify display fabrication and operation.
The number of leads can be reduced in a number of ways. For example, half of the electrodes (either all the positive electrodes or all the negative electrodes) can be eliminated by using one common electrode for the positive or negative electrode. The common electrode may be discrete islands of conductive material (often transparent for optical displays) electrically connected together either inside or outside the display cell. More convenient from a fabrication point of view is a large, usually planar electrode that serves all of the opposite discrete electrodes.
Although the use of one single electrode as either the positive or negative electrode reduces the number of leads substantially (from 96 to 49 for the 7 character, 7 element display discussed above), cross-talk effects are substantially increased with corresponding reduction in optical contrast and sharpness of the display.
Further reduction in the number of leads and individual circuits needed for a display are obtained using a multiplexing technique. A typical circuit arrangement is given in FIG. 2, assuming again a 7 character, 7 segment display as described above. The number of leads is now reduced to 14. Indeed, for most modern display circuits, some sort of multiplexing arrangement is used so that the number of leads is kept at a reasonable level. However, a multiplexing arrangement substantially increases the cross-talk effect in the display and thereby reduces optical contrast and sharpness of the display.
Techniques for reducing or eliminating cross-talk effects are described in many references including the following: "Liquid-Crystal Displays-Electro-Optic Effects and Addressing Techniques", by L. A. Goodman, RCA Review 35, page 613 (Dec. 1974); "Liquid Crystal Matrix Displays", by B. J. Lechner, Proc. IEEE 59, page 1566 (1971; "Multiplexing Liquid-Crystal Displays", by Paul Smith, Electronics, May 25, 1978, page 113; "Multiplexing LED Displays", by George Smith, Australian Electronics Eng. Sept. 1972, page 36; and "Multiplexing LED Digital Display Circuitry", by George Smith, Electronics Australia, April 1975, page 50.
These references describe various multiplexing systems where cross-talk effects are reduced or eliminated by adding additional circuit elements (diodes, capacitors, etc.). It is desirable to reduce or eliminate cross-talk effects without adding additional circuit elements which increase cost and complicate fabrication techniques.